Inverters can convert electrical power of an energy generation unit from direct current to alternating current and feed it into an energy supply grid, for example an AC voltage grid, and/or feed it in directly to an electrical load for the operation thereof. Generators, in particular, can be used here as the energy generation unit, said generators generating electrical power from renewable sources, for example photovoltaic or wind generators.
An inverter can be connected to an AC voltage grid in various ways. Particularly at powers from a few kilowatts, inverters can be used that feed into a three-phase AC voltage grid by means of a three-phase output. A three-phase AC voltage grid of this kind has three so-called outer conductors or phases, which are often referred to as L1, L2 and L3. In addition, a neutral conductor (N conductor for short, or simply N) and optionally a protective conductor (PE conductor for short, or simply PE) may be present, wherein N and PE can also be embodied as a combined PEN conductor. A four-conductor grid of this kind is referred to, in particular, as a TN system and is used, in particular, in the low-voltage range, for example in distribution grids.
In contrast to a TN system, a so-called IT system does not have a neutral conductor and thus constitutes a three-conductor grid. IT systems of this kind can be used, in particular, in energy generation installations with a plurality of inverters, which are connected to a superordinate distribution or transmission grid by means of a common grid connection point. In view of the costs for installation, it may be advantageous for an operator of an extensive energy generation installation with a plurality of inverters and a correspondingly large number and lengths of lines between the inverters and the grid connection point to form a 3-conductor grid, for example an IT grid.
The operation of an inverter is dependent on the actual embodiment of the AC voltage grid to which the inverter is connected. For example, a measurement of output voltages that is necessary for regulation of the output currents of the inverter may be done between the outer conductors and a reference-ground potential, wherein the reference-ground potential can be formed by the neutral conductor, when said conductor is present and connected to the inverter. If this is not the case, another potential, for example a ground potential, is to be selected as the reference-ground potential. Furthermore, protective measures such as residual current detection can be designed differently depending on whether a flow of current through a neutral conductor is possible or not. Parameterization of the regulation of the output currents and the generation of output currents with specific properties, for example asymmetrical output currents for stabilizing the AC voltage grid, depend on the presence of a neutral conductor and the connection thereof to the inverter.
Essentially two different types of operational management procedure of an inverter are known, which depend on the specific design of the energy generation installation comprising the inverters, in particular on the specific cabling within the energy generation installation or on the specific connection of the inverter to the AC voltage grid. An inverter, which is intended to be able to be used universally, can comprise, in principle, an output terminal, which is also provided for connection to an N conductor, wherein said connection does not necessarily have to be present and, in particular, is not used when the inverter is intended to feed into a 3-conductor grid without a neutral conductor.
EP 2251702 A1 discloses a wiring test apparatus for testing the wiring of an inverter, in which the inverter is configured for connection to a low-voltage three-phase grid having conductor voltages with respect to a neutral conductor on the grid side, wherein a measuring circuit is provided to measure the conductor voltages. In the case of a missing neutral conductor connection, voltage asymmetry is created on account of an additional element, in particular a capacitor, which is connected on the output side of the inverter between a phase and the neutral conductor, said voltage asymmetry being able to be determined by measuring the conductor voltages. Consequently, a fault message can be output in the event of a missing neutral conductor connection. The additional component generates additional costs and impairs the electromagnetic compatibility of the inverter by virtue of the fact that the properties of an output filter likewise arranged on the output side of the inverter is negatively influenced.
US 2009/0296289 A1 discloses a method for identifying a fault in a converter of a motor controller, wherein the input side of the converter is connected to an AC voltage grid and the output side of the converter has a three-phase connection for a load. An input-side and/or an output-side ground fault can be detected by virtue of the fact that the inverter attempts, based on the actuation thereof, to feed in a zero-phase sequence system current to the AC voltage grid. A fault signal is emitted when a zero-phase sequence system current is detected in the phase conductors connected to the inverter. A connection of the converter to a possible neutral conductor of the AC voltage grid is not disclosed.